1. Description of the Prior Art
Among the various types of fuel cell systems are those which include subassemblies of two bipolar plates between which are supported electrodes and an acid electrolyte in a matrix. The subassemblies, herein referred to as fuel cells, are oriented one atop, or aside another, and electrically connected in series, or otherwise, to form what is commonly referred to as a fuel cell stack. Process channels are provided in the bipolar plates, adjacent to the electrodes, so that fuel flows through one set of process channels and an oxidant flows through another set of channels.
In typical bipolar plate fuel cell constructions, the fuel and oxidant streams flow on opposite sides of the electrolyte matrix at 90.degree. angles to one another, commonly referred to as a cross-flow geometry. The oxidant and fuel channels are symmetrically and evenly distributed, that is, a repeating selected channel shape is evenly spaced across the cells. It has now been recognized that as the fuel or oxidant traverses these channels, the desired electrochemical reaction takes place, accordingly depleting the fuel or oxidant. The fuel and oxidant are fresh at the respective inlet ends of the process channels, and progressively degrade to a minimum at the cell exit. The progressive depletion accordingly affects the electrochemical reaction rate across the cell. Further, as a result of the approximate 90.degree. cross-flow orientation, a typical rectangular cell presents one corner exposed to both fresh fuel and fresh oxidant, while the diagonally opposite corner is exposed to both depleted fuel and depleted oxidant. Thus, the entry corner tends to develop a higher EMF than the exit corner, and this change occurs gradually across the entire cell. At specific locations across the cell an imbalance is thus created between the degree of depleted fuel and the depleted oxidant.
The resulting uneven reaction distribution is undesirable for several reasons, including unbalanced effects upon current, resistance and temperature throughout an individual fuel cell and a fuel cell stack. For example, if depletion of either oxidant or fuel is complete to the extent that the electrochemical reaction locally ceases, that position within the cell tends to reverse the reaction, substantially effecting cell output and overall system efficiency. While excessive amounts of fuel and oxidant can be passed through the process channels to eliminate this possibility, there still remains an uneven reaction distribution and additionally an efficiency penalty from the mechanical power required to pump the reactants.
It is therefore desirable to provide fuel cell systems which alleviate the uneven reaction occurring across the fuel cells.